1. Field of the Invention
The present invention relates to a liquid container capable of stably supplying a liquid such as ink contained therein, a liquid using device, a printing apparatus, and a method of manufacturing the liquid container.
2. Description of the Related Art
An ink tank for directly supplying ink to a print head capable of ejecting ink has a negative pressure generation mechanism that generates a negative pressure to be applied to the ink. The negative pressure generated by the negative pressure generation mechanism is set in an appropriate range that is large enough to balance with a retaining force of an ink meniscus formed at ink ejection portions in the print head to prevent leakage of ink from the ink ejection portions and which also allows ink ejections from the print head.
Such a negative pressure generation mechanism is formed, for example, by installing in an ink tank a porous material such as sponge capable of soaking and holding ink and generates an appropriate negative pressure by an ink retaining force of the porous material. Another example of the negative pressure generation mechanism has a bag-like member formed of an elastic material such as rubber capable of producing a tensile force that tends to expand a volume of the bag. The bag-like member is filled with ink and, through its tensile force, applies a negative pressure to the ink. Still another example has a bag-like member formed of a flexible film and engages a spring with an interior or exterior of the bag-like member to urge the flexible film in a direction that expands the volume of the bag, thereby applying a negative pressure to the ink in the bag-like member.
With these negative pressure generation mechanisms, however, the negative pressure generated tends to increase as the amount of ink remaining in an ink tank (bag-like member) decreases. When the negative pressure exceeds a predetermined level, ink can no longer be supplied stably to a print head. This gives rise to a possibility of the ink tank becoming unfit for use before the ink in the ink tank is completely consumed.
This is explained in the following example. U.S. Pat. No. 4,509,062 discloses an ink tank which comprises a hermetically closed resilient bag member and a spring member installed in the bag member. The resilient bag member directly accommodates ink and is deformable according to the amount of ink contained therein. The spring member urges the bag member in a direction that expands its volume. With this ink tank, the negative pressure in the bag member is basically such as will balance with a spring force of the spring member. Thus, as the bag member deforms to reduce its volume to match the ongoing consumption of ink and the inner spring is compressed, the negative pressure in the bag member increases. As a result, the negative pressure may increase in excess of an appropriate range that allows normal ink ejections from a print head, making it impossible for an adequate meniscus to be formed at the ink ejection portions of the print head or to supply ink stably to the print head. In this case, not all of the ink volume in the bag member cannot be used.
There is also an ink tank which produces a negative pressure by taking advantage of an elasticity of an ink accommodating bag member itself whose material and shape are determined appropriately. The bag member is formed flat so that its inner space vanishes when the ink contained therein is completely used up. This kind of bag member, however, has a limitation on the shape. If an ink tank is constructed of a box-like case accommodating a bag member, the bag member even when loaded with ink does not assume a shape that perfectly fits in the case, degrading an ink accommodation efficiency with respect to an overall ink tank space. Even with this bag member, when the ink is about to be used up, the negative pressure is so high as will cause a performance degradation in supplying ink to the print head or make the ink ejection operation of the print head unstable.
To prevent the level of negative pressure generated by the negative pressure generation mechanism from exceeding a predetermined level, the following adjust mechanisms have been proposed.
For example, U.S. Pat. Nos. 5,917,523 and 5,600,358 disclose an adjust mechanism which has a ball arranged in a tube vent in an ink tank (container) so that when a negative pressure in the ink tank increases, air is taken into the ink tank to prevent a negative pressure increase. In this adjust mechanism, the tube vent (boss) communicating an interior of the ink tank with the outside has a plurality of protruding ribs formed on its inner wall. A ball with an outer diameter smaller than that of the boss is fitted inside the boss so that it is in contact with the protruding ribs. As a result, a roughly ring-shaped orifice is formed between the ball and the boss. A size of this orifice is so set that a small amount of ink is held as a liquid seal in the orifice by its capillary attraction. When the negative pressure in the ink tank approaches an allowable limit of the operation range of the print head, the negative pressure overcomes the ink capillary attraction in the orifice, breaking the liquid seal and allowing air to enter into the ink tank through the orifice.
Japanese Patent Application Laid-open No. 6-183023 (1994) describes another adjust mechanism for preventing a negative pressure rise. This adjust mechanism is used in a negative pressure generation mechanism which comprises a plate with a hole and a plate with a protrusion, both arranged to face each other in an ink bag of resilient sheet, and a spring member arranged between these plates. When the ink bag contracts as a result of a reduction in the remaining volume of ink and the inner negative pressure exceeds a predetermined value, the adjust mechanism causes the protrusion of one plate to fit into the hole of the other plate, thus separating the holed plate from the resilient sheet to allow air to be introduced into the ink bag. With this adjust mechanism, after air is drawn into the ink bag, the holed plate and the resilient sheet are brought into intimate contact with each other, preventing an ink leakage by an ink meniscus retaining force or a liquid seal between them.
These negative pressure adjust mechanisms disclosed in U.S. Pat. Nos. 5,917,523, 5,600,358 and Japanese Patent Application Laid-open No. 6-183023 (1994), however, all require a plurality of parts in the air take-in portion, rendering the construction that much complicated.
The adjust mechanism disclosed in U.S. Pat. Nos. 5,917,523 and 5,600,358 forms a hermetically closed system as an ink accommodation space through a balance between an ink meniscus force (liquid seal) in a ring-shaped orifice and a negative pressure produced by a spring. Although the mechanical construction is relatively simple, this adjust mechanism lacks a stability in maintaining the hermetically closed system. That is, the liquid seal in the orifice may be broken depending on various conditions, resulting in a leakage of accommodated ink. The conditions that may cause an ink leakage include a pressure difference between the inside and outside of the ink tank, a reduction in ink viscosity due to temperature rise, an inadvertent impacts on or fall of the ink tank during handling, and an acceleration to which the ink tank is subjected during a main scan in a serial printing apparatus in which the ink tank is moved in the main scan direction along with the print head. The liquid seal is easily affected by humidity variations, such as dry atmosphere. The humidity variations therefore make an air introducing operation unstable, which in turn may lead to a performance reduction in supplying ink to the print head and to a degraded quality of printed images.
To eliminate these problems, the adjust mechanism disclosed in U.S. Pat. Nos. 5,917,523 and 5,600,358 provides an inlet maze connecting to an annular boss. The inlet maze is considered to function as an ink overflow container and secure a humidity gradient. The provision of the inlet maze, however, complicates the construction. Further, since the other end of the inlet maze (maze-like path) communicates with open air at all times, the ink unavoidably evaporates to some degree through this inlet maze.
There is another problem. When ink in the ink tank is used up, outer air rushes into the ink tank through the ring-shaped orifice to eliminate the negative pressure in the ink tank. At this time, the inrush air may cause the ink remaining in the print head and the ink tank to leak out of nozzles or through the ring-shaped orifice in which the meniscus has been broken.
Further, in the adjust mechanisms disclosed in U.S. Pat. Nos. 5,917,523, 5,600,358, and Japanese Patent Application Laid-open No. 6-183023 (1994), a liquid-sealed opening is provided in the ink container (i.e., an ink tank in U.S. Pat. Nos. 5,917,523 and 5,600,358; and an ink bag in Japanese Patent Application Laid-open No. 6-183023 (1994)) to directly, introduce the atmosphere. When ink in the ink container is almost running out and a volume of air in the ink container is larger than that of ink, if the atmosphere is introduced into the ink tank through the opening, the maintenance of a meniscus in the liquid-sealed opening of the container and in the ink nozzle openings of the print head may become incomplete. This in turn may cause an ink leakage and render the introduction of air incomplete. Depending on a variety of conditions the liquid seal in the opening may be broken, resulting in an early introduction of air before the pressure in the ink container reaches a predetermined value or, conversely, a leakage of ink. The conditions leading to unwanted air introduction or ink leakage include a pressure difference between the inside and outside of the ink container, temperature variations, impacts on and fall of the ink tank during handling, and an acceleration to which the ink tank is subjected during a main scan in a serial printing apparatus in which the ink tank is moved in the main scan direction along with the print head. These conditions change depending on the design of the print head and ink tank and a physical property of ink, so it is difficult to properly design a shape and dimensions of the opening.
The negative pressure adjust mechanisms using the liquid seal described above may also reduce a degree of freedom of design in the printing apparatus.
That is, it is difficult to form the liquid seal portion separate from the ink tank and then removably mount it on the ink tank. If the liquid seal portion is formed separate from the ink tank, when it is directly mounted on the ink tank or indirectly connected to the ink tank through a tube or the like, complex processing or a special construction considering a pressure difference between the inside and outside of the ink tank is required in order to form a good meniscus in the liquid seal portion. Where the liquid seal portion is provided remote from the ink tank and connected to it through a tube, the tube needs to be filled with ink in order to form a meniscus in the liquid seal portion. The introduction of air through the liquid seal portion forces the ink in the tube back into the ink tank. Refilling the tube with ink after the air introduction requires complicated processing or construction.
In the adjust mechanism disclosed in Japanese Patent Application Laid-open No. 6-183023 (1994), since air is introduced through a small clearance between a thin plate member and a flexible sheet, a capillary attraction produced by a liquid entering that clearance changes a force required to separate the holed plate and the flexible sheet. As a result, the negative pressure level at which the air introduction is executed may become unstable. Further, when a pressure of gas (air) in the ink bag increases as the temperature increases, the flexible sheet must be deformed to virtually increase the inner volume of the ink bag to alleviate the increasing inner pressure. Therefore, the flexible sheet member is formed of an easily deformable material with a very low stiffness.
However, low-rigidity materials used for such a flexible sheet generally have a small thickness and a high gas permeability, so air can easily pass through it. Thus, if ink is stored in the ink bag for a long period of time, a large volume of air, so large as cannot be dealt with by a buffer function originally intended to absorb an expanded portion of gas (air) in the ink bag, enters into the ink bag, rendering the buffer function ineffective. It is therefore necessary to use a very expensive material deposited with a metal vapor to meet both of the requirements of a low rigidity and a low gas permeability.